This invention relates to improvements in motor control circuits and in particular to pulse width modulation (PWM) control of multiple phase brushless motors. It also relates to methods of monitoring the operation of a motor.
Control systems for PWM controlled electric motors, especially DC motors, generally need to measure the current through the windings or phases of the motor and this can either be done by means of separate current sensors for each of the phases, or by means of a single current sensor that is placed in the circuit so as to measure the total instantaneous current flowing between a D.C. power supply and the bridge circuit and motor combination. In a single current sensor system, the multiple motor phase currents are derived by offsetting the PWM patterns of the switches which apply the required voltage to each phase, and sampling the current sensor at appropriate points.
The measured currents are typically converted into the stationary d-q frame and then combined with a current demand signal, also in the d-q frame, indicative of the current that is demanded from the motor, to produce an error signal. The demand current is itself typically generated as a function of the torque demanded from the motor. The error signal represents the difference between the current that is demanded and the actual measured current. The error signal is fed to a controller which produces a set of voltage demand signals, also typically in the d-q frame, representative of the voltage to be applied to the motor that will best drive the error signal towards zero. The d-q voltages are then converted into PWM signals for the motor phases depending on which PWM strategy is used. The controller therefore acts to vary the PWM phase voltages in order to try to constantly minimise the magnitude of the error signal thereby ensuring that the motor current is as close as possible to the demanded current.
In a practical system the controller will comprise a PI or PID or other type of feedback controller. The frequency of response, or bandwidth, of the feedback controller determines the maximum frequency of change in the motor current that the controller will try to track. Frequencies above this will be largely inconsequential to normal operation of the feedback loop. This normal bandwidth should be set to a high enough level to permit the current in the motor to be tracked but not so high as to track noise and for the circuit to become unstable. Generally the controller responses will be tuned to the time constant of the motor so as to ensure an optimum tracking of the demanded motor current. This is achieved through suitable selection of the gains in the controller, ensuring it is neither over nor under damped and can track the expected changes in current.
Motor drive circuits using feedback control and PWM are well known in the art. For example, WO 2007/072033, discloses a typical system and the teaching of that document is incorporated herein by reference.